1. Field of the Invention
The present invention relates to a plastic optical fiber and a production method thereof.
2. Description of the Related Art
Plastic optical fibers, hereinafter referred to as POFs have advantages such as lightweight, excellent workability, and low production cost compared to quartz based POFs due to being formed of plastics. The POFs also have other advantages such as easily increasable diameter, and high bending tolerance due to high flexibility. However, on the other hand, these advantages increase the optical transmission losses in the POFs, making the POFs not suitable for optical transmittance for long distance. The POFs attract attention as light transmission media for a short distance such as for domestic use, vehicle-mounted use, or the like in which the transmission losses are not concerned.
The POF is constituted of a core and a clad. The clad is formed on the outer periphery of the core and has a lower refractive index than the core. Light propagates through the core, and the clad prevents light leakage therefrom. Thus, the POF is constituted of sections having different properties. The POF functions as the light transmission medium by totally reflecting the light at an interface between the core and the clad having different refractive indices.
The POFs are broadly divided into graded index (GI) type, step index (SI) type, and multistep index (MSI) type according to refractive index profiles. Recently, GI POFs attract the most attention. The GI POFs are light transmission media which achieve high-speed transmission with low transmission losses by preventing light scattering and irregular reflections.
The POFs including the GI POFs are produced by heat-drawing preforms with the use of heating devices. The preform is constituted of a core section which is a precursor of the core, and a clad section which is a precursor of the clad. To form the preform, a cylindrical clad section is prepared, and then a polymerizable compound which is formed into a core section is filled into a hollow portion of the clad section. The cylindrical clad section containing the polymerizable compound is rotated and polymerized to fabricate the preform having the core section inside the clad section. The POF is obtained by heat-drawing the preform.
Since the POFs are constituted of plastic, they exhibit excellent bending tolerance. However, on the other hand, bending causes transmission losses (hereinafter referred to as bending losses). The bending losses significantly lower the function of the POFs as the optical transmission media. The POFs with the reduced bending losses are suggested in the following: a GI POF having at least two layers each of which containing fluorine-containing polymer as a matrix, and the outer layer contains the fluorine-containing polymer whose refractive index is lower than the outermost portion of the inner layer by 0.001 or more (for instance, see Japanese Patent Laid-Open Publication No. 08-304636); a plastic optical fiber cable which has cavities between an outer periphery of a POF and a coating layer (for instance, see Japanese Patent Laid-Open Publication No. 09-218327); a plastic optical fiber cable having a coating layer formed of resin foam around an outer periphery of a POF (for instance, see Japanese Patent Laid-Open Publication No. 09-236735); a POF provided with two coating layers having different elastic moduli around its outer periphery (for instance, see Japanese Patent Laid-Open Publication No. 11-337781); and a POF having an inner layer formed of non-crystalline fluorine-containing polymer without C—H bonds and an outer layer formed of fluorine-containing polymer (for instance, see U.S. Patent Application Publication No. 2002/0009276, corresponding to Japanese Patent Laid-Open Publication No. 2002-071972). Further, a method for forming a POF with a reduced bending loss is suggested. In this method, a non-crystalline thermoplastic resin composition is coated around an outer periphery of a preform having a certain refractive index profile by extrusion-coating. Thereafter, the coated preform is heat-drawn into a POF (for instance, see Japanese Patent Laid-Open Publication No. 2000-098144.)
In the case the crystalline fluorine-containing polymer is used for the outer layer, spherocrystals formed in the inner wall of the outer layer makes the surface of the inner wall rough, causing the light scattering at the interface between the inner layer and the outer layer. As a result, the bending loss cannot be reduced. On the other hand, in the case non-crystalline fluorine-containing polymer is used for forming the outer layer, it is inferior in solvent resistance to a monomer. As a result, it becomes difficult to form a core which will be the optical transmission path with a monomer inside the outer layer. In the case the cavities are formed between the POF and the protective layer, it is difficult to control shapes, sizes and conditions of the cavities. In the case the resin foam layer is formed, it is difficult to control foam conditions in the resin foam. Therefore, diameter fluctuations of the POF and exacerbation of non-circularity may occur.
The coating layers having different elastic moduli are effective for reducing structural imperfection caused by bending. However, it is difficult to reduce the bending losses namely the light leakage and/or the light scattering caused at the interface between the clad and the core. In the case the diameter of the core which is the optical transmission path is extremely small, it becomes difficult to connect the POFs. As a result, the workability is reduced. Therefore, a production method of the POF by which the bending loss of the POF is reduced without lowering the productivity is needed.